Bituminous strips for roof covering material are typically formed of a support such as foil, fabric or non-woven material (or web), which support can optionally be bitumen-impregnated, and which support is further coated at least on one side with a bituminous covering. The surface of the bituminous covering is provided with a conventional separating means for the purpose of preventing the strip from bonding to itself when being in a coiled-up condition. The separating means can also fulfill further functions such as providing ultra-violet protection at the uppermost layer of the roofing material.
To improve their mechanical properties, the bituminous covering materials of the past have been modified with bitumen-compatible/polymers. These polymers increase the range of plasticity of the material and also increase the viscosity of the melt. When rubber and rubber-like polymers are added to such materials, both the softening point is raised and cold flexibility is improved. Furthermore, the elasticity of the bituminous material is improved thereby. Such polymers are usually based on double bond-containing monomers. These double-bond containing polymers, however, suffer from the drawback of low aging resistance to ultraviolet (uv) radiation. It is known to add polylefins such as polyethylene in order to advantageously affect aging behavior, but as a rule no improvement in cold flexibility is achieved, and, moreover, the elasticity is only slightly improved. The plasticity range of known elastomer-modified polymer bitumen is approximately between +125.degree. and -35.degree. C. and as regards the olefin-modified ones, between about +150.degree. and -15.degree. C. The plasticity range therefore is approximately equal but is shifted by about 25.degree. K. This is as important a factor in selecting a roofing strip as its bonding and fusing behavior or the resistance to chemicals and aging.
DD No. 215,559 proposes to improve the dimensional stability of bitumen mixtures even at higher temperatures by adding radiation-crosslinked polymers, in particular polyethylene. The degree of crosslinking should be selected in such a manner that the crosslinked polymer still can adequately dissolve in the bitumen, because highly crosslinked polymers must be considered being fillers capable of only slightly affecting the properties of the bituminous materials. Crosslinking raises the molecular weight of the polymer, which in known manner positively affects the softening point but negatively affects the rupture point and penetration (See STRASSE UND AUTOBAHN 1986, pp 3-9).
The bitumen mixtures of DD No. 215,559 as shown by the description, are meant most of all for road construction. This is also apparent from the bitumen selected in the Examples, where bitumen types B50 and B80 are used, which are unsuitable as roof covering materials.